A moisture-permeable waterproof material is a highly functional material in which two contradictory properties, namely moisture permeability for allowing air or water vapor to pass and waterproofness for preventing the penetration of liquid such as raindrops, are balanced well, and has become popular due to application not only to ski wear or rainwear but also to many types of sportswear for climbing, golf, marine sports, etc. Techniques for realizing such a moisture-permeable waterproof material have been steadily developed since the 1960s and then accelerated owing to the development of Gore-Tex in the mid and late 1970s, and various developments are still being made to improve the performance thereof.
A moisture-permeable waterproof material may be classified according to the processing method thereof into a coated type in which a film is formed and a laminated type in which a thin film is bonded, and the coated type may be divided into (i) a wet-coated type in which a polyurethane coating layer having fine pores is formed on a fabric in water and (ii) a dry-coated type in which a uniform polyurethane film layer is formed on a fabric by applying a resin composition including an organic solvent having high volatility to a predetermined thickness on the fabric and then performing thermal treatment to volatilize the organic solvent. Furthermore, depending on the principle whereby the moisture-permeable waterproof function is exhibited, it may be classified into a microporous type comprising fine pores, a nonporous hydrophilic type, a nanoweb type, and a shape-memory type.
With regard to the moisture-permeable waterproof material, patent techniques using a laminating process include Japanese Patent Application Publication No. Hei. 5-124144, which discloses a moisture-permeable waterproof sheet manufactured by laminating a polyethylene porous film and a nonwoven fabric comprising a heat-bondable fiber using heat and pressure, and Japanese Patent Application Publication No. Hei. 3-213581, which discloses a moisture-permeable waterproof fabric having high wear resistance obtained by laminating, on a fabric, a moisture-permeable film comprising poly-amino-acid-based polyurethane containing 0.1% or more of a powder composed of L-lysine and an organic acid, and additionally, a variety of dry/wet coating processes are disclosed in Japanese Patent Application Publication Nos. Hei. 4-249142, 4-146275, 7-258971, 5-78984, and 8-13352.
The manufacturing methods disclosed in the above conventional techniques are very complicated, and the resulting moisture-permeable waterproof film or membrane has non-uniform pores, making it impossible to solve the problem of low moisture permeability. Furthermore, in the case where the organic solvent is used during the manufacturing process, the dye of the fabric is released due to the organic solvent, undesirably causing contamination.
Meanwhile, polytetrafluoroethylene (PTFE), which is commonly known to be a material for Gore-Tex, has high moisture permeability, chemical resistance and lightfastness because of the uniform formation of fine pores in the film, and is thus very favorably used despite the high price thereof. However, PTFE is poor in mechanical fatigue strength and washing durability and involves the risk of releasing environmentally harmful materials such as PFCs (perfluorinated and polyfluorinated compounds), FTOHs (fluorotelomer alcohols), PFCAs (perfluorinated carboxylic acids), PFOS (perfluorinated sulfonate), PFOA (perfluorooctanoic acid), etc.
Although polyurethane, which has high moisture permeability and may be subjected to various processing methods, has been mainly used to date as the moisture-permeable waterproof material, polyurethane suffers from poor washing durability when applied to clothes, thereby decreasing hydrostatic pressure. In particular, when the pore size of the film is increased in order to ensure air permeability as high as that of PTFE, the mechanical strength of the material may be drastically decreased. Moreover, in the case where polyurethane is directly applied on a fabric through a wet or dry process, the resin solution may penetrate into the fabric, and thus the tactile sensation of products may become hard, and furthermore, limitations are imposed on exhibiting functionality such as moisture permeability, water resistance, etc.